There is an absolute need to develop biodegradable lubricants for use in machinery which, in the event of leakages through damage, would cause pollution of the environment. This problem has been discussed in many commissions and initiatives. Standards have been defined, including for example VDMA 24 568 or Swedish Standard SS 15 54 34. In addition, product labels, such as “Blauer Engel (Blue Angel)” or “Weisser Schwan (White Swan)”, have been devised as a particular mark of environmentally safe products.
Besides high biodegradability, such diverse lubricants as engine oil, turbine oil, hydraulic fluid, transmission oil, compressor oil and the like have to satisfy extremely stringent criteria, such as high viscosity index, good lubricity, high oxidation sensitivity, high thermal stability or other comparable criteria. Development work is currently being carried out on the Europe-wide EU-Ecolabel Standard. Criteria include, inter alia, low toxicity, biodegradability and at least 50% renewable raw materials.
Ester-based lubricating oils are known per se and have been in use for sometime (see Ullmanns Enzyklopädie der technischen Chemie, 3rd Edition, Vol. 15, 1964, pp, 285-294). Common esters are reaction products of dicarboxylic acids with medium-chain alcohols, such as 2-ethyl hexanol for example, or reaction products of polyols, such as trimethylol propane for example, and fatty acids, such as oleic acid for example, or a mixture of n-octanoic and n-decanoic acid. Relatively low viscosities at low temperatures, and hence improved handling behavior at relatively low temperatures have been described in particular for esters with branched alkyl chains. Such esters are, for example, reaction products of Guerbet alcohols with dicarboxylic acids which are described as a component of lubricants in EP 489 809.
In addition, DE 2302918 describes ester oils of polyols with branched acids which show improved viscosity temperature behavior in relation to lubricants based on mineral oils. Moreover, it is known from U.S. Pat. No. 5,488,121 for example, that esters of Guerbet alcohols and Guerbet acids, so-called di-Guerbet esters, lead to lubricants distinguished by high oxidation stability and good viscosity/temperature properties.
It is also known that esters with completely saturated side chains show increased oxidation stability. A particular problem arises when, in addition to high oxidation stability and low low-temperature viscosity, the lubricants are expected to show improved compatibility with sealing materials. This problem applies above all to readily biodegradable hydraulic fluids. The known lubricants based on linear esters with high oxidation stability are saturated in character, but cause softening of the usual sealing materials. Conversely, unsaturated esters emanating from oleic acid, for example, show better behavior towards sealing materials, but greatly reduced oxidation stability. Particular problems arise with such sealing materials as NBR (nitrile/butyl rubber) and hydrogenated variants thereof (HNBR).
An Article by Torbacke et al. (“Synthetic Lubrication” (2005), 22(2), 123-142) investigates the compatibility with sealing materials of lubricants based on esters, such as monoesters, polyol esters, diesters and complex esters, by comparison with mineral oils.
There is still a need for improved lubricants with high biodegradability.